1. Field of the Invention
This invention relates to an organic optical component made from a polyamide, its copolymer, or a polyimide. The organic optical component of this invention has excellent transparency, mechanical strength and heat resistance as well as a low birefringence, which is useful as, for example, a substrate for an optical disk, a pick-up lens, a plastic substrate for a liquid cell and a prism. This invention also relates to a novel fluorine-containing polyamide with a low dielectric constant, excellent transparency, low hygroscopicity and excellent processability.
2. Description of the Related Art
Inorganic glasses have a number of excellent physical properties such as excellent transparency and a reduced optical anisotropy, and thus has been used in various fields. The glasses, however, have problems such as fragility due to their heavy weight and a poor productivity, leading to recent intensive attempts for developing a transparent polymer as a substitute for an inorganic glass.
A transparent polymer such as poly(methyl methacrylate) and polycarbonate has excellent transparency, mechanical properties such as shock resistance, processability and moldability, which has been, therefore, used in various applications such as transparent components of a car and a lens, as an alternative to an inorganic glass. These transparent polymers, however, have inadequate heat resistance; for example, even a polycarbonate belonging to an engineering plastic has a glass-transition temperature of about 150.degree. C., leading to limiting its use at an elevated temperature.
Meanwhile, an optical disk on which information such as sounds, images and texts is recorded and reproduced using a laser beam, has been being rapidly extended in its use. In an optical disk used as an information recording medium, a laser beam passes through the disk body. Thus, the disk is required to be, of course, transparent, and is strongly required to be optically homogeneous for reducing reading errors of an information. For example, when using a conventional polymer such as polycarbonate and poly(methyl methacrylate), there occurs a problem that a residual stress generated by some factors such as thermal stress, molecular orientation and volume variation near a glass-transition temperature generated by cooling and fluidizing processes of a resin during casting a disk substrate, may cause a birefringence when a laser beam passes through the disk substrate. Large optical heterogeneity due to the birefringence may become a fatal defect for an optical component such as an optical disk substrate because it may cause significant problems such as reading errors of a recorded information. Hence, it has been desired to provide an optical component, typically an optical disk substrate, made from a material with more advanced optical characteristics, e.g., a low birefringence and excellent transparency and heat resistance.
For solving the above problems, JP-A 63-314235 has disclosed a low-birefringent polycarbonate from a spiro compound such as a homopolymeric polycarbonate of spirobiindanol or a copolymeric polycarbonate of spirobiindanol and bisphenol-A. Although having a low birefringence, the former polycarbonate is practically problematic due to its poor transparency and mechanical strength, while in the latter polycarbonate, increase of bisphenol-A improves heat resistance, i.e., reduction of the glass-transition temperature, transparency and mechanical strength, but increases the birefringence, leading to limiting its applications as an optical component. Thus, it has been strongly desired to solve these conflicting problems.
Furthermore, polyimides are well known as an engineering plastic with high heat resistance. Although having good heat resistance, polyimides have a high birefringence. For example, the polyimide disclosed in JP-A 8-504967 may be used as an optical material, but has a birefringence of at least 0.01 level which is not adequately low. Furthermore, according to "PHOTOSENSITIVE POLYIMIDE--Fundamentals and Applications", edited by KAZUYUKI HORIE and TAKASHI YAMASHITA TECHNOMIC PUBLISHING COMP., p.300 (1995), commercially available polyimides have a birefringence of at least 0.1; even a special fluorinated polyimide indicates a birefringence of 0.01 level. Thus, these may significantly improve heat resistance, but as described above, considerably limit their use as an optical component.
In addition, aromatic polyamides are also known as heat resistant resins like polyimides. However, optical properties, particularly a refractive index and a birefringence, have not been described very much for aromatic polyamides, and thus, substantially no data on the properties are available.
Aromatic polyamides have been expected to be extensively used in different fields, because they have a variety of excellent physical properties, especially heat resistance. Although the polyamides developed to date have excellent properties, they have some drawbacks such as poor processability and high water absorptivity. Thus, for solving the problems, JP-A 5-310919 has suggested a polyamide prepared by condensation polymerization of the diamine represented by formula (A) with an aromatic dicarboxylic acid in an organic solvent, and a process for preparing it. ##STR3##
This polyamide has excellent processability represented by a glass-transition temperature of 177 to 195.degree. C. and a 5% weight-loss temperature of 484 to 505.degree. C., as well as a low water absorption of 0.70 to 0.75%, without losing the good properties inherent in a polyamide. However, polyamides including the above-mentioned generally have a relatively higher dielectric constant, which inhibits their application to an electronic material.